This is an article about how disputes over virtual world items, such as virtual money, Second Life islands, and even sex beds, can inform property law generally. Rights in these virtual world items, like rights in software and many other intangible assets, are transferred by standard-form agreements that are often designated as licenses. For many readers, virtual worlds need no definition; it has been hard to read a major newspaper in the past several years without encountering an article about virtual worlds. In the past several years, Second Life and other virtual worlds were featured in numerous articles in major American newspapers, including the New York Times, the Washington Post, and the Wall Street Journal. Virtual worlds have captured the attention of legal and other scholars. The legal literature tends to focus on the application of real world laws to the virtual environment. Some have discussed how our property laws should apply in virtual worlds; others have questioned whether virtual worlds need their own governance institutions. In this article, I will take another approach. Rather than asking whether real world laws can or should apply to virtual worlds, I will discuss the ways in which the study of virtual worlds can contribute to real world law. Specifically, I will explain what the study of virtual world assets can do for property law. In this paper, I argue that virtual world assets are significant because they graphically illustrate the different rights that persons can hold in an intangible asset. Once we see that intangible assets encompass the very same rights that are embodied in tangible assets, we can understand that the law should not permit the unfettered customization of property rights in intangible assets by standard form agreements, just as the law does not permit the unlimited customization of property rights in tangible assets and real property. My thesis is that a study of virtual world assets can help us understand why the numerus clausus principle should be more rigorously applied to rights in intangible assets and that the numerus clausus can, in turn, assist us interpreting the standard-form agreements that convey rights in these assets.